Conventionally, a sheet finisher is known which is installed downstream of an image forming apparatus such as a copier, printer or multi-functional peripheral (MFP) and performs finishing such as punching or stitching of printed sheets.
The functions of the sheet finisher are increasingly diversified. A sheet finisher (middle-folding device) is proposed which has a folding function to fold a part of the sheet and a function of saddle-stitching and middle-folding to staple a central part of the sheet and then fold the sheet at the central part, in addition to the functions of punching and stitching.
In the case of such middle-folding, it is preferable to form a satisfactory fold in order to reduce the thickness of the folded booklet. To this end, it is preferable to nip and fold sheets one by one between a pair of rollers. However, folding sheets one by one in this manner takes time and efficient folding cannot be performed. Meanwhile, when plural sheets are superimposed and folded at a time, efficient folding can be performed. However, it is difficult to form a satisfactory fold on each of the plural sheets.
In view of the foregoing problems of the conventional technique, the invention is to provide a folding device capable of forming a satisfactory fold on each sheet and performing efficiency folding, an image forming apparatus using this device, and a folding method for the same.